1. Field of the Invention
The present invention relates to a thermal system liquid ejection head used in an inkjet printer and the like and to a liquid ejection device such as an inkjet printer and the like including the liquid ejection head, and relates to a technology for realizing a flow path structure without uneven ejection by minimizing a flow path failure caused by intrusion of dusts and the like and occurrence of bubbles.
2. Description of the Related Art
Heretofore, in a liquid ejection head used in a liquid ejection device represented by, for example, an inkjet printer, there is known a thermal system making use of expansion and contraction of generated bubbles and a piezo system making use of fluctuation of the shape and the volume of a liquid chamber.
In the thermal system, heating elements are disposed on a semiconductor substrate, bubbles are generated to a liquid in a liquid chamber, the liquid is ejected from nozzles disposed on the heating elements as liquid droplets, and the liquid droplets are landed on a recording medium and the like.
FIG. 25 is an outside perspective view of this type of a conventional liquid ejection head 1 (hereinafter, simply referred to a head 1) In FIG. 25, a nozzle sheet 17 is bonded on a barrier layer 3, and FIG. 25 shows the nozzle sheet 17 by disassembling it.
FIG. 26 is a sectional view showing a flow path structure of the head 1 shown in FIG. 25. Note that this type of the flow path structure of the liquid ejection device is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2003-136737.
In FIGS. 25 and 26, a plurality of heating elements 12 are disposed on a semiconductor substrate 11. Further, the barrier layer 3 and the nozzle sheet 17 are sequentially laminated on the semiconductor substrate 11. A member, in which the heating elements 12 as well as the barrier layer 3 are formed on the semiconductor substrate 11, is called a head chip 1a. A member, in which the nozzle sheet 17 is bonded on the head chip 1a, is called the head 1.
The nozzle sheet 17 has nozzles 18 (holes for ejecting liquid droplets) which are disposed to position on the heating elements 12. Further, the barrier layer 3 is disposed on the semiconductor substrate 11 so as to be interposed between the heating elements 12 and the nozzles 18 so that liquid chambers 3a are formed between the heating elements 12 and the nozzles 18.
As shown in FIG. 25, the barrier layer 3 is formed in a comb shape when viewed in a plan view so that three sides of the heating elements 12 are surrounded thereby. With this arrangement, liquid chambers 3a are formed with only one sides thereof opened.
Individual flow paths 3d are formed to the open portions and communicate with a common flow path 23.
The heating elements 12 are disposed in the vicinity of a side of the semiconductor substrate 11. In FIG. 26, a dummy chip D is disposed on the left side of the semiconductor substrate 11 (head chip 1a), thereby the common flow path 23 is formed by a side surface of the semiconductor substrate 11 (head chip 1a) and a side surface of the dummy chip D. Note that any member may be used in place of the dummy chip D as long as it can form the common flow path 23.
As shown in FIG. 26, a flow path sheet 22 is disposed on the surface of the semiconductor substrate 11 opposite to that on which the heating elements 12 are disposed. As shown in FIG. 26, an ink supply port 22a and a supply flow path 24 are formed to the flow path sheet 22. The supply flow path 24 has an approximately concave sectional shape so as to communicate with the ink supply port 22a. The supply flow path 24 communicates with the common flow path 23.
With the above arrangement, ink is supplied from the ink supply port 22a to the supply flow path 24 and the common flow path 23 as well as enters the liquid chambers 3a through the individual flow path 3d. When the heating elements 12 are heated, bubbles are generated on the heating elements 12 in the liquid chambers 3a, thereby a part of the liquid in the liquid chambers 3a is ejected from the nozzles 18 by trajectory force when the bubbles are generated.
Note that, in FIGS. 25 and 26, the shapes of the respective components are exaggeratedly shown ignoring the actual shapes thereof for the sake of easy understanding. For example, the thickness of the semiconductor substrate 11 is about 600-650 μm, and the thickness of the barrier layer 3 is about 10-20 μm.
In the head 1 of the conventional technology described above, a problem arises in that, first, the liquid fails to be ejected from the nozzles 18 and is supplied to the flow paths in an insufficient amount because dusts and the like come into the flow paths and the nozzles 18.
Dust and the like float and move freely in an ordinary space. Accordingly, they drop in the liquid and exist therein as dusts and the like. In liquid ejection devices such as inkjet printers and the like, however, the nozzles 18 may be clogged with dusts and the like because the structure thereof is such that a liquid is ejected from nozzles 18 having a diameter of several microns.
To cope with the above problem, at present, parts are rinsed with a liquid and the like containing a less amount of dusts and the like in a working atmosphere, for example, in a clean room, and the like in a manufacturing process.
Further, in design, filters must be disposed in the flow paths of the liquid ejection device at several positions to eliminate dusts and the like.
In particular, since an increase in the number of nozzles as in a line head increases the probability of failed injection of a liquid from the nozzles 18, dusts and the like must be more strictly managed, from which a problem of an increase in cost arises.
Further, bubbles may be generated in the liquid as a result of an increase in the temperature of the head 1, from which a problem arises in that the liquid is ejected in an insufficient amount due to the bubbles.
Although the common flow path 23 and the individual flow paths 3d are exemplified as the positions where bubbles are generated, the liquid is ejected unevenly even if they are generated in any of the positions.
FIG. 27 is a photograph showing the state of bubbles remaining in a common flow path 23.
In FIG. 27, the nozzle sheet 17 is formed of a transparent member so that the state of the bubbles in the nozzle sheet 17 can be observed.
In FIG. 27, a filter is disposed in the common flow path 23. The filter is disposed to prevent invasion of dusts and the like in the individual flow paths 3d, and composed of column-shaped pillars disposed along the common flow path 23.
As shown in FIG. 27, the amount of the liquid supplied to the individual flow path 3d is reduced in the region (the region surrounded by a dotted line) in which bubbles remain in the common flow path 23. Accordingly, the amount of ejection of the liquid is reduced, thereby an unevenly ejected liquid having a reduced density appears in a wide region.
Note that, as a reason why the ejected state of the liquid is affected by bubbles, it is contemplated that the ejection of the liquid itself is affected by pressure generated in the ejection and a reaction which corresponds to the pressure and is determined by the liquid in the vicinity of the liquid chamber 3a, the barrier layer 3, and the existence of the bubbles.
Further, bubbles may come into the vicinities of the inlets of the individual flow paths 3d and into the individual flow paths 3d. FIG. 28 is a photograph showing the state of bubbles remaining in the inlet of the individual flow path 3d. In FIG. 28, the nozzle sheet 17 is formed of a transparent member likewise in FIG. 27.
In this case, even if bubbles are small in size, they have a significant influence because they exist in a small space. That is, the amount of ejection of the liquid is more reduced than the state shown in FIG. 27. Further, only the amount of ejection of the liquid from the nozzle 18 corresponding to the individual flow path 3d into which bubbles come is reduced, the liquid becomes conspicuous as a stripe.
When the bubbles described above are generated once, they are adhered to the common flow path 23 and the individual flow paths 3d or reciprocatingly move between the common flow path 23 and the individual flow paths 3d and do not simply disappear even if the liquid is repeatedly ejected. Further, since the liquid is supplied into the liquid chambers 3a passing among the bubbles, insufficient ejection characteristics are often maintained fixedly.
Note that it is confirmed that bubbles disappear when an ejecting operation is stopped and the temperature of the liquid is lowered by being left for a long period of time, from which it can be found that the bubbles in this case are generated by the evaporation of the liquid.
In contrast, since a portion surrounded by a bubble is composed of a gas, it has a bad coefficient of thermal conductivity, thereby the heat of a heating portion is liable to be accumulated in the portion because it is not cooled by the liquid. As a result, a problem arises in that the bubble is expanded.
Since there is a tendency that bubbles are particularly liable to be generated when the center of the heating element 12 is displaced from that of the nozzle 18, it is also contemplated that the bubbles generated on the heating element 12 remain without being effectively used for ejection.
Further, bubbles may come into the liquid chambers 3a and the nozzles 18. FIG. 29 is a photograph showing the state in which a gas comes into the liquid chambers 3a from nozzles 18.
In FIG. 29, although a filter (triangular-prism-shaped pillars are disposed different from the column-shaped pillars in FIG. 27) is disposed in the common flow path 23, since the spaces between the pillars of the filter are clogged with bubbles which are combined with each other and grown, the liquid cannot move to the liquid chambers 3a side.
When the movement of the liquid from the common flow path 23 to the liquid chambers 3a is checked by the bubbles, the balance of the meniscuses of the nozzles 18 is liable to be broken. In this state, impact waves from adjacent nozzles trigger a gas to come into the liquid chamber 3a of the nozzle 18. That is, since the pressure of the liquid in the head 1 is set lower than atmospheric pressure, when the balance of meniscuses is broken, the liquid moves backward to the common flow path 23 side and cannot be ejected.
Further, there is also a problem in that the liquid is ejected unevenly by the impact waves in ejection coupled particularly with the existence of bubbles. Note that, in the thermal system, the pressure in ejection is more significantly changed as compared with the piezo system.
The following two problems are exemplified as problems caused by impacts in ejection.
First, impact waves trigger to cause bubbles to be drawn from adjacent liquid chambers 3a. 
It is contemplated to increase the intervals between the pillars of the filter to avoid this problem. In the case, however, since the size of dusts and the like passing through the filter is increased, large dusts and the like are liable to come into the individual flow paths 3d. 
Second, since the impact waves are transmitted to adjacent nozzles 18, the meniscuses of the nozzles 18 are vibrated to thereby cause uneven liquid ejection. When bubbles are generated or remain, they are encountered with the impact waves, thereby the bubbles are liable to be drawn and the uneven liquid ejection is liable to be caused.
Incidentally, in a serial system in which an image can be formed by overlapping dots (overlapped writing), even if there are one or two nozzles which eject the liquid unevenly, the uneven liquid ejection can be recovered by making it inconspicuous by the overlapped writing. In contrast, in a line system, in which image formation is completed by ejecting droplets once and the overlapped writing cannot be executed in principle, the uneven liquid ejection cannot be recovered different from the serial system.